Continuous analysis for copper concentration

ABSTRACT

An analyzer for directly and continuously measuring the concentration of soluble copper in an aqueous solution by adding potassium thiocyanate thereto in a dilute solution to form cupric thiocyanate. The cupric thiocyanate passes through an electrochemical cell where it is quantitatively reduced to cuprous thiocyanate. The resultant electric current is directly proportional to copper concentration and is measured to provide an indication thereof.

United States Patent [191 Morrow et al.

[ Dec. 31, 1974 CONTINUOUS ANALYSIS FOR COPPER CONCENTRATION [75] Inventors: James J. Morrow, Norristown; Leo L. Dailey, Philadelphia, both of Pa.

[73] Assignee: Fischer & Porter Co., Warminster,

[22] Filed: Dec. 11, 1973 [21] Appl. No.: 423,758

Related U.S. Application Data [62] Division of Ser. No. 782,212, Dec. 9, 1968 abandoned.

[52] US. Cl 204/1 T, 23/230 R [51] Int. Cl. G0ln 27/46 [58] Field of Search 204/1 T, 195 R; 23/230 R,

[5 6] References Cited UNITED STATES PATENTS 2,782,151 2/1957 Suthard 204/1 T Ova/vase 2,851,655 9/1958 Haddad 204/195 R 3,043,764 7/1962 Harvey 204/195 R 3,298,934 l/ 1967 Angeleri 204/1 T 3,421,850 1/ 1969 Peterson et a1. 204/1 T 3,449,233 6/1969 Morrow 204/ 195 Primary Examiner-T. Tung [5 7 ABSTRACT An analyzer for directly and continuously measuring the concentration of soluble copper in an aqueous solution by adding potassium thiocyanate thereto in a dilute solution to form cupric thiocyanate. The cupric thiocyanate passes through an electrochemical cell where it is quantitatively reduced to cuprous thiocyanate. The resultant electric current is directly proportional to copper concentration and is measured to provide an indication thereof.

5 Claims, 7 Drawing Figures 1400 61m: v2.0a Asmwmw- C436 F5960- my 2, W

PATENTEU m3 1 m4 sum 10F 21 q Mk g vnw N Vm in L PAH-1mm secs 1 1914 SHEET 2 BF 3 gi HP CONTINUOUS ANALYSIS FOR COPPER CONCENTRATION RELATED APPLICATION:

This application is a division of the copending application Ser. No. 782,212 filed Dec. 9, 1968, now abandoned. This invention relates generally to copper analysis and more particularly to a technique and apparatus for carrying out a continuous analysis of copper concentration in a flowing stream or pipe line.

In the copper industry, standard laboratory analytical methods applied to samples taken from a copperbearing stream afford only a spot check of copper content. In modern process control systems, it is generally necessary to provide a continuous rather than an occasional or intermittent indication of copper content in order to effect automatic control which is immediately responsive to any change in content.

Accordingly, it is the main object of the invention to provide a quantitative analyzer for continuously measuring soluble copper in aqueous solution.

More specifically, it is an object of this invention to directly and continuously measure copper concentration in a process stream or tank by electrochemical reduction of a solution of copper salt, the current flow involved in the reduction being recorded to afford a reading of copper content.

Briefly stated, these objects are accomplished by adding potassium thiocyanate to a solution of cupric copper in a dilute solution to form cupric thiocyanate. When the cupric thiocyanate is passed through an amperometric type of electrochemical cell having the proper potential impressed on the electrodes, it is quantitatively reduced to cuprous thiocyanate. The electric current flow induced in the cell by the reduction is directly proportional to copper concentration. While the presence of ferric or ferrous iron in the stream interferes with the reading, such interference may be removed by the use of fluoride ions.

For a better understanding of the invention, as well as other objects and further features thereof, reference is made to the following detailed disclosure to be read in conjunction with the accompanying drawing wherein:

FIG. 1 is a schematic illustration of one preferred embodiment of an analyzer in accordance with the invention for measuring copper concentration;

FIG. 2 schematically illustrates the amperometric cell assembly incorporated in the analyzer;

FIG. 3 is a graphical showing of the electrochemical characteristics of the cell;

FIG. 4 graphically shows the interfering effect of ferric iron on the current flow in the cell;

FIG. 5 graphically shows the interfering effect of ferrous iron on current flow in the cell;

FIG. 6 shows the effect of the addition of fluoride ion on current flow in the cell; and

FIG. 7 schematically illustrates another preferred embodiment of the analyzer incorporating means to prevent interference by reason of the presence of iron in the stream.

ANALYZER FOR DISSOLVED COPPER SALT We shall first consider the construction and operation of an analyzer adapted to measure copper concentration in a stream containing only dissolved copper salts and acids.

Referring now to FIG. 1, a sample diverted from a copper-bearing stream enters the copper analyzer at a sample inlet 10 at a rate which, in one working embodiment, is about 0.5 gallon/minute. Most of the sample is returned to the stream through the sample outlet 11. A small portion of the sample, as determined by a manually-adjustable sample flow-control valve 12, is admitted into an overflow chamber 13, the overflow going through a line 14 to a drain 15 (approximately five gallons per day).

From overflow chamber 13, the sample is drawn at a precisely metered rate by a micro-flow sample pump 16 through a heat exchanger 17, this being accomplished by means of a tube 18 which passes through the exchanger and is immersed in the overflow chamber.

In heat exchanger 17, the sample flowing through tube 18 is cooled by water admitted therein through line 19. If the sample temperature is less than 120 F, the heat exchanger is not essential. Sample pump 16 delivers the sample which is drawn from overflow chamber 13 and thereafter cooled into the upper inlet 20 of a dilution chamber 21 by way of flexible line 22.

A reagent micro-flow pump 23 delivers potassium thiocyanate solution from a reagent supply tank 24 through flexible line 25 to the lower inlet 26 of dilution chamber 21. The concentration of potassium thiocyanate (KSCN) in the reagent solution is such that the ratio of KSCN to copper by weight in the electrochemical cell is approximately :1. At the bottom of supply tank 24 is a pressure sensitive switch 24A to actuate an alarm circuit when the tank level falls below a predetermined level.

Dilution water of drinking quality enters the analyzer through inlet pipe 27 and passes through a dechlorinating filter 28 to remove any residual chlorine which would otherwise interfere with proper copper measurement. The dechlorinated water then passes through pipe 29 into a differential regulator 30 and a fixed orifree 31 from which it is conducted through pipe 19 into the heat exchanger 17 where it serves to cool the sample passing therethrough.

The regulator 30 and orifice 31 function to produce a constant flow rate which, in practice, may be about 300-350 ml/minute. After cooling the sample in the heat exchanger 17, the water then flows through line 32 into dilution chamber 20.

Dilution chamber 20 is provided with a baffle arrangement 33 to produce a mixing action between the sample copper solution, the reagent and the water. The stream leaving dilution chamber 20 through line 34 now contains cupric thiocyanate and excess KSCN as well as other reaction products of no consequence, from which it passes into the amperometric electrochemical cell assembly which isshown in FIG. 2.

In the electrochemical cell, the stream enters a flow splitting block 35 by means of which any portion or all of the flow is directed by way of tube 36 through the measuring cell 37. Cell 37 comprises a measuring electrode 38 of high purity gold which is rotated by motor 39 at high speed at a rate in the order of 1,500 RPM.

Surrounding measuring electrode 38 is a stationary counter-electrode 40 of an aluminum alloy. By means of a suitable electronic circuit housed in box 41, the electrical potential of the measuring electrode 38 is maintained between +0.10 volt d-c and 0.20 volt d-c versus S.C.E. At this potential, the cupric thiocyanate which is formed when potassium thiocyanate is added to a solution of cupric copper in dilute solution is re- Cu (SCN) e CuSCN (SCNY 2. Aluminum Electrode An electrical current is caused to flow in the cell due to the reduction of copper from the cupric to the cuprous form, the intensity of current flow being directly proportional to the concentration of copper in the sample stream. The current flow is measured by a suitable circuit including a temperature compensating network having a thermistor 42 to sense the stream temperature. The network serves to correct for variations in dilution water temperature. The millivolt output signal of the cell measuring circuit is applied to a suitable recorder (not shown) to provide a continuous record of copper concentration. It will be obvious that the output signal produced by the analyzer which is proportional to copper concentration may be used in a process control system to maintain copper concentration at a desired level or for other purposes.

The polarographic current/voltage waves developed by the amperometric cell is shown in FIG. 3. Curve A represents the current flow developed in the cell at gram/liter of copper. It will be seen that this current flow is at the constant level of about 0.15 milliamperes when the potential of the gold measuring electrodes vs. S.C.E. (volts) is between about +0.2 to 0.2, the working range of the cell.

When the copper content is doubled (40 g/l), then the resultant current flow, as indicated by curve B, is at about 0.3 milliamperes. Hence the intensity of current flow is directly proportional to copper concentration.

INTERFERENTS In copper process streams, iron in ferric or ferrous form is often present. Either form of iron causes interference in the copper measurement. The interfering effect of ferric (Fe iron is shown graphically in FIG. 1. In the polarographic curve A, there is shown the current produced in the electrochemical cell where Cu and Fe are present in the same sample.

In curve B, the current for Cu only is shown, while in curve C the current for Fe only is shown. Even though the Cu concentration is the same on curves A and B, the readings are distinctly different and hence not a proper index of copper-concentration.

The interference effect of ferrous (Fe iron is shown in FIG. 5, the curve A being current due to Cu while curve B is the current due to Cu and Fe. Here again the copper concentration in curves A and B are the same, but the cell current levels are different and hence not a proper index of this concentration.

Ferric iron produces a positive error by being reduced to ferrous iron at the gold measuring electrode at about the same cell potential at which copper is reduced, thus generating additional current flow and a higher output signal. It has been found that the addition of fluoride ion to the sample causes formation of a ferric fluoride complex Fe F: the reduction potential of which is a far more negative value than that of copper.

The effect on the polarographic wave is shown in FIG. 6. Curve A is the copper reduction wave, the working potential range of the analyzer being shown by cross hatching. Curve B is the FE F reduction wave. It

will be seen that the ferric reduction wave is moved beyond the working potential of the analyzer, thereby effectively obviating the interference.

The addition of fluoride ion alone will not prevent the negative interference due to the presence of ferrous iron. However, if nitric acid (l-INO is added to the sample before fluoride is added, all iron present in the sample will be changed to the ferric form and the interference removed in the manner previously explained.

ANALYZER WITH IRON INTERFERENCE REMOVAL Referring now to FIG. 7, there is shown a copper concentration analyzer in which provision is made to remove iron interference. Those elements in the system which correspond to elements contained in the system in FIG. 1 are identified by like reference numerals.

In addition to KSCN, the reagent in tank 24 contains ammonium bifluoride (NH HF in solution which when added to the sample stream in dilution chamber 21, provides a fluoride/iron weight ratio of about /1. A small amount of hexametaphosphate is also added to this reagent to prevent precipitation of calcium salts from the dilution water by fluoride.

A second reagent supply tank 43 is provided to hold a 10 percent by volume solution of Nitric Acid (HNO Micro-flow pump 44 functions to deliver the I-lNO solution from tank 43 through line 45 to a reaction chamber 46. Also fed into reaction chamber 46 is the sample stream delivered by the sample micro-pump 16.

In reaction chamber 46, the copper sample and the HNO solution are intermingled by a mixing rotor driven by a motor 47. The intermingled sample and HNO goes from reaction chamber 46 to the upper inlet 20 of dilution chamber 21. The analyzer system is in all other respects thesame as that shown in FIG. 1.

OPERATING FACTORS In the amperometric cell, the concentration of copper should be in the range of O to 4 mg/liter. By proper selection of motor speed and tubing size, the flow rate of the micro-flow sample pump 16 can be varied to cover a broad range of copper concentration in the stream to be analyzed. The concentration of copper in the cell is expressed by the following equation:

Accordingly,

.025 ml 40 mg Cu min Inl 1000 ml 2.85 mg Cu c 350 ml H O liter liter min Q If one decreases the sample pump speed to 1/4 RPM with the same tubing size (i.d. l/64 inch) and the same dilution water flow rate, the analyzer range can be extended to concentration up to 200 grams per liter of copper.

l/4 RPM at l/64 inches i.d. 0.00625 ml/min =f .00625 m1 200 mg Cu O min ml X 1000 rnl 3.57 mg 0 350 ml H O liter liter min By using a higher speed pump motor and larger pump tubing, the range of the analyzer can be lowered to concentrations in the order of less than 1 gram/liter of copper.

Using a pump with l/l6 inches i.d. tubing at 10 RPM, the sample pump flow rate is 3.5 ml/min.

Assuming 3.00 mg/liter C and f 350 ml/minute,

350 m1 3 mg min liter 300 mg a 3.5 ml liter Copper Concentration in accordance with the inventien: itw laeataia q a e we y sha s find m ifications may be made therein without, however, departing from the essential spirit of the invention as defined in the annexed claims.

We claim:

1. A technique for continuously measuring the concentration of cupric copper in an aqueous solution comprising the steps of adding potassium thiocyanate to a sample of said cupric copper solution in a dilute solution to form cupric thiocyanate, passing said cupric thiocyanate through an electrochemical cell operating at a voltage acting to reduce it to cuprous thiocyanate thereby inducing an electric current in the cell which is directly proportional to copper concentration, and measuring the current to indicate the copper concentration.

2. A technique as set forth in claim 1, wherein said cupric copper is in an aqueous solution having iron present therein, the further step of adding fluoride ion to the sample to form a ferric fluoride complex to prevent the presence of the iron from affecting the measurement.

3. A technique as set forth in claim 1, further including the step of cooling said sample of cupric copper solution before it is diluted.

4. A technique as set forth in claim 1, further including the step of adding ammonia bifluoride to said potassium thiocyanate to prevent the presence of iron from affecting the measurement.

5. A technique as set forth in claim 1 wherein said dilute solution of a sample of said cupric copper is produced by adding dechlorinated water thereto. 

1. A TECHNIQUE FOR CONTINUOUSLY MEASURING THE CONCENTRATION OF CUPRIC COPPER IN AN AQUEOUS SOLUTION COMPRISING THE STEPS OF ADDING POTASSIUM THIOCYANATE TO A SAMPLE OF SAID CUPRIC COPPER SOLUTION IN A DILUTE SOLUTION TO FORM CUPRIC THIOCYANATE, PASSING SAID CUPRIC THIOCYANATE THROUGH AN ELECTROCHEMICAL CELL OPERATING AT A VOLTAGE ACTING TO REDUCE IT TI CUPROUS THIOCYANATE THEREBY INDUCING AN ELECTRIC CURRENT IN THE CELL WHICH IS DIRECTLY PROPORTIONAL TO COPPER CONCENTRATION, AND MEASURING THE CURRENT TO INDICATE THE COPPER CONCENTRATION.
 2. A technique as set forth in claim 1, wherein said cupric copper is in an aqueous solution having iron present therein, the further step of adding fluoride ion to the sample to form a ferric fluoride complex to prevent the presence of the iron from affecting the measurement.
 3. A technique as set forth in claim 1, further including the step of cooling said sample of cupric copper solution before it is diluted.
 4. A technique as set forth in claim 1, further including the step of adding ammonia bifluoride to said potassium thiocyanate to prevent the presence of iron from affecting the measurement.
 5. A technique as set forth in claim 1 wherein said dilute solution of a sample of said cupric copper is produced by adding dechlorinated water thereto. 